A pulse radar comprises a transmitter and a receiver. The transmitter transmits a finite-length radio wave pulse which is scattered or reflected from a medium or solid target. The receiver may be used to measure the reflected signal's power or Doppler shift or both. Samples are taken from the received signal so that the power can be calculated by squaring the samples taken. The power measurement yields information about the radar reflectivity of the target or medium; the reflectivity is often referred to as the scattering or reflecting cross section. Doppler shift estimates are based on the signal's autocorrelation function values which are obtained by multiplying the samples by samples delayed in accordance with a desired delay value. Typical pulse radar applications include weather radars in which power measurements are used to determine rainfall and delay measurements are used to determine wind speeds. Surveillance radars measure the location and movement of aircraft, ships and other targets. Remote sensing radars are used in satellites, airplanes or helicopters to measure various features of the Earth's surface. In addition, there exist scientific radar apparatuses for ionospheric measurements (at heights of 70 to 1000 km) and measurements of the lower atmospheric layers (mesosphere-stratosphere-troposphere, or MST, radars). Sonar is a radar-like apparatus used for underwater measurements based on acoustic waves instead of radio waves, and a lidar is basically a radar where laser light is used instead of radio waves. An object or medium, whose location and/or movement is measured by means of the radar, may be called a target in general. The three-dimensional space in which the measurement is carried out is called the measurement volume.
As an example, let us consider weather radar measurements employing uniform transmission of pulses. The pulse repetition frequency (PRF) is chosen according to the particular measurement.
When measuring radar reflectivity, a low enough PRF (300 to 500 Hz) is used so that a transmitted pulse will leave the measurement volume before the next pulse is transmitted. Thus, the received signal will only contain responses from one measurement volume, yielding an unambiguous reflectivity measurement result. The maximum measurement range r.sub.max may then be calculated according to the equation EQU r.sub.max =cT/2, (1)
where c is the velocity of the radio waves (acoustic waves in the case of sonar; light in the case of lidar) in the medium, and T is the time separation between adjacent pulses, i.e. the inverse of the pulse repetition frequency. For example, for radio waves transmitted through the air at a repetition frequency of 500 Hz the time separation between adjacent pulses is 2 ms and the maximum measurement range is 300 km.
When measuring wind speed, the time separation between adjacent pulses determines the maximum velocity v.sub.max that can be unambiguously measured. It may be defined as EQU v.sub.max =(.lambda./4).multidot.PRF, (2)
were .lambda. is the wavelength. At 5.6 GHz, which falls within the widely used radar frequency range called the C band, we get v.sub.max =0.0134 PRF, when the velocity is given in meters per second and the repetition frequency is given in Hz. A typical PRF is 1 kHz, in which case the maximum measurable velocity is 13.4 meters per second.
Equations (1) and (2) show that as the PRF increases, the maximum velocity increases but the maximum unambiguous range decreases and vice versa. In real-life measurements it is not always possible to simultaneously measure both the velocity and the range, at least not accurately. In the literature, this phenomenon is referred to as the range-Doppler dilemma or the range-velocity ambiguity (cf. e.g. Doviak and Zrnic, "Doppler radars and weather observations," Chapter 3.6, Academic Press, 1993). No solution was known to this problem in September 1994 (COST 75 Weather Radar Systems, International Seminar, Brussels, Belgium, Sep. 20-23, 1994, EUR 16013 EN, 1995; U.S. Department of Commerce, NOAA, Notice for Proposal Solicitation for a solution to "Doppler Dilemma").
Below it is mentioned known attempts to solve the problem described above. U.S. Pat. No. 3,935,572 discloses a system employing four parallel measurement channels. U.S. Pat. No. 3,987,443 discloses a radar in which the PRF is changed from time to time. In U.S. Pat. No. 4,328,495 each pulse comprises phase-coded sub-pulses. U.S. Pat. No. 4,924,231 discloses a method for processing a great quantity of transmitted signals and their echoes in order to find the best correlation. U.S. Pat. No. 5,027,122 discloses a method for improving Doppler measurement by means of signal processing. In U.S. Pat. No. 5,247,303 the pulses are divided into frames, and at least one pulse in each frame is purposely distorted. U.S. Pat. No. 5,276,453 discloses a method based on the use two different signal frequencies. The invention disclosed in U.S. Pat. No. 5,583,512 uses a common two-dimensional correlator for the simultaneous determination of range and Doppler shift. U.S. Pat. No. 5,621,514 discloses a system which employs light pulses instead of radio frequencies and in which the received signal is processed to determine the Doppler shift. U.S. Pat. No. 5,659,320 deals with sonar without delving into the contradiction between the velocity and reflectivity measurements. U.S. Pat. No. 5,724,125 describes another light-pulse based measurement arrangement with multiple repetitions and signal processing in a system of linear equations. PCT application document WO 96/00909 and the corresponding U.S. Pat. No. 5,442,359 disclose arithmetic methods for processing a received signal.
If the continuity of the measurement is not important, the problem described above may be solved using a method based on so-called multipulse codes, disclosed e.g. in a publication called "Multiple-pulse incoherent-scatter correlation function measurement" by Farley, Radio Science, 7, pp. 661-666. In said method a small number of pulses (ordinarily 3 to 6 pulses) are transmitted such that all pulse intervals are unequal in addition to being (small) multiples of one interval. Transmission is then stopped and reception started. Transmission is not started again until the last pulse of the previous pulse train has traveled very far, typically about 2000 km. This method has been applied especially to ionosphere radar measurements in which the area of interest lies very far away from the radar. In that case it is only useful that echoes coming from near the radar are not received due to the late start of the reception. The method is not suitable to weather radar and short-range surveillance radar applications since data are obtained only from areas located far away from the radar.
In a multipulse code based measurement, the shortest pulse interval determines the greatest measurable velocity in accordance with Equation (2). The total length of the transmitted multipulse code limits the greatest determinable autocorrelation function delay value, but unambiguous measurement of velocity is in principle possible at ranges of arbitrary length. Power measurement is not unambiguous but yields a sum of powers reflected from a plurality of different ranges. The data received in one power measurement thus constitute the combined data from a number of ranges, said number equaling the number of pulses in the code. This kind of power measurement was once regarded as useless, but subsequently a method for utilizing the measurement has been disclosed in the article "The use of multipulse zero lag data to improve incoherent scatter radar power profile accuracy" by Lehtinen and Huuskonen, J. Atmos. Terr. Physics, 48, pp. 787-793, which is incorporated herein by reference. The efficiency of the multipulse measurement has been improved by the use of so-called alternating codes as in the article "A new modulation principle for incoherent scatter measurements" by Lehtinen and Haggstrom, Radio Science, 22, pp. 625-634, which is also incorporated herein by reference; alternating codes appear to be considerably more effective than the previously known multipulse codes.